1. Field of the Invention
The present invention relates to a display device and a driving method thereof, and in particular to a display device which employs a time gray scale method.
2. Description of the Related Art
In recent years, a self-luminous display device having a pixel formed of light emitting elements such as a light emitting diode. As a light emitting element used for such a self-luminous display device, a light emitting diode (also referred to as an OLED (Organic Light Emitting Diode), an organic EL element, an inorganic EL element, and an electroluminescence (also referred to as an EL element) is attracting attentions and starting to be used for an EL display (an organic EL display, an inorganic EL display, or a display including an element containing organic and inorganic substances). A light emitting element such as an OLED which is a self-luminous element is advantageous in that visibility of pixels is high, a backlight is not required, response is fast, and the like as compared to a liquid crystal display. The luminance of a light emitting element is controlled by a value of a current flowing therethrough.
There are a digital gray scale method and an analog gray scale method as a driving method for controlling a light emission gray scale of such a display device. In the digital gray scale method, a light emitting element is controlled in a digital manner to be turned on and off, thereby a gray scale is expressed. In the analog gray scale method, on the other hand, there are a method for controlling light emission intensity of a light emitting element in an analog manner and a method for controlling light emission time of a light emitting element in an analog manner.
In the case of the digital gray scale method, there are only two states: light emission and no light emission. Therefore, only two gray scale levels can be expressed. In view of this, multi-level gray scale levels are expressed by using another method in combination. In that case, a time gray scale method is often used.
The time gray scale method is a method for expressing a gray scale by controlling the time and the number of light emission. That is, one frame period is divided into a plurality of subframe periods in each of which the time and the number of light emission are weighted. A gray scale is expressed by providing differences in the total amount of the weight (total number and time of light emission) between gray scales. When employing the time gray scale method, it is known that a display defect occurs which is called pseudo contour or the like, for which a countermeasures are being considered (see Patent Documents 1 to 7).
[Patent Document 1]
Japanese Patent No. 2903984
[Patent Document 2]
Japanese Patent No. 3075335
[Patent Document 3]
Japanese Patent No. 2639311
[Patent Document 4]
Japanese Patent No. 3322809
[Patent Document 5]
Japanese Patent Laid-Open No. hei 10-307561
[Patent Document 6]
Japanese Patent No. 3585369
[Patent Document 7]
Japanese Patent No. 3489884